🐛 fix(archive): group multi-sourced posts by year

Closes #251.

templates/archive.html

@@ -25,7 +25,16 @@
             {%- set_global all_posts = all_posts | concat(with=section_item.pages) -%}
         {%- endfor %}
 
-        {% for year, posts in all_posts | group_by(attribute="year") %}
+        {# Group posts by year. #}
+        {% set posts_by_year = all_posts | group_by(attribute="year") %}
+        {% set years = [] %}
+        {% for year, ignored in posts_by_year %}
+            {% set_global years = years | concat(with=[year]) %}
+        {% endfor %}
+
+        {# Iterate over sorted & reversed years (newest to oldest). #}
+        {% for year in years | sort | reverse %}
+            {% set posts = posts_by_year[year] %}
             {% if posts | length > 0 %}
             <li>
                 <h2 class="listing-title">{{ year }}</h2>